Impedance matching circuit and method thereof

ABSTRACT

A circuit comprises an amplifier circuit and a trimming circuit. The amplifier circuit includes an operational amplifier. The operational amplifier has a first input configured to receive input signals, and the operational amplifier also has a second input and an amplifier output. One of the first input or the second input is a negative input. The trimming circuit is coupled to the amplifier output. The trimming circuit includes a termination resistor coupled in parallel with at least one trimming resistor. The termination resistor is coupled to a first switch in series, and the trimming resistor is coupled to a second switch in series. The amplifier output is connected back to the negative input through the first switch.

TECHNICAL FIELD

Exemplary embodiments of the present invention relate to a semiconductorproduct and method thereof, and more particularly, to a trimming circuitin a communications system.

BACKGROUND

Semiconductor devices implemented by integrated circuits are widely usedfor various electronic applications or communications systems. In thetransmission of information or data via electrical signals, transmittingas much of the power as possible from power source to load is oftendesirable. However, undesired variations in behavior of semiconductordevices resulting from manufacturing processes may result in impedancemismatches, which may cause large power consumption and limit theaccuracy of the circuit behavior. Accordingly, it is typically desirableto reduce the impact of impedance mismatches on the performance ofcircuits and to maximize the power transfer. There are a variety oftechniques to reduce the impact of impedance mismatches. Among them,impedance matching is the most commonly used technique. Impedancematching may involve attempts to make an output impedance of a source,such as a power amplifier (PA) of a communications system, and an inputimpedance of a load, such as an antenna or a wired communicationsmedium, attain a desired relationship so that maximum power transfer,maximum voltage transfer, maximum efficiency, minimum signalreflections, and so forth, are achieved.

There are various devices, for example, termination resistors,transformers, inductors and capacitors or combination of them, that maybe used between power source and load that perform impedance matching.In many conventional impedance matching circuits, termination resistorsmay reside outside the semiconductor device or may be integrated intothe circuit boards to lower the cost. In the later example, a trimmingcircuit is by far the most common technique to more precisely matchsource impedance to load impedance.

FIG. 1 is a schematic diagram of a conventional impedance matchingcircuit 100 utilizing a trimming circuit. As is shown, the circuit 100includes an amplifier circuit 102 and a trimming circuit 104. Thetrimming circuit 104 includes a termination resistor R_(S0) and aplurality of trimming resistors R_(S1), R_(S2) . . . R_(SN) in parallel.The trimming resistors are coupled to respectivemetal-oxide-semiconductor (MOS) transistors S₁, S₂, . . . S_(N) used asswitches in series. For example, the trimming resistor R_(SN) is coupledto the MOS transistor S_(N) in series. Because the inherent nonlinearcharacteristics of the MOS transistors are not included in the negativefeedback circuit, the MOS transistors may transfer the nonlinearitiesthrough the trimming resistors to the load, and hence cause distortionsof the input signals which may degrade the linearity of the outputsignal Vout.

BRIEF SUMMARY

According to one exemplary embodiment of the invention, a circuit forcorrecting the linearity of the output signal comprises an amplifiercircuit and a trimming circuit coupled to the amplifier output. Theamplifier circuit includes an operational amplifier. The operationalamplifier has a first input configured to receive input signals, and asecond input and an amplifier output. One of the first input or thesecond input is a negative input. The trimming circuit includes atermination resistor coupled in parallel with at least one trimmingresistor. The termination resistor is coupled to a first switch inseries, and the trimming resistor is coupled to a second switch inseries. The amplifier output is connected back to the negative inputthrough the first switch.

According to one exemplary embodiment of the invention, a circuit forcorrecting the linearity of the output signal comprises an amplifiercircuit and a trimming circuit coupled to the amplifier output. Theamplifier circuit includes an operational amplifier. The operationalamplifier has a first input configured to receive input signals, and asecond input and an amplifier output. One of the first input or thesecond input is a negative input. The trimming circuit includes atermination resistor coupled in parallel with at least one trimmingresistor. The termination resistor is coupled to a first switch inseries, and the trimming resistor is coupled to a second switch inseries, such that a ratio of a resistance of the first switch to aresistance of the second switch approximately equals a ratio of aresistance of the termination resistor to a resistance of the trimmingresistor.

According to one exemplary embodiment of the invention, a method forcorrecting the linearity of the output signal comprises providing anoperational amplifier, the operational amplifier having a first inputconfigured to receive input signals, and the operational amplifier alsohaving a second input and an amplifier output. One of the first input orthe second input is a negative input. The method further comprisesproviding a trimming circuit coupled to the amplifier output. Thetrimming circuit includes a termination resistor coupled in parallelwith at least one trimming resistor. The method further comprisesproviding a first switch coupled to the termination resistor in series,and a second switch coupled to the at least one trimming resistor inseries, such that a ratio of a resistance of the first switch to aresistance of the second switch approximately equals a ratio of aresistance of the termination resistor to a resistance of the trimmingresistor. The amplifier output is connected back to the negative inputthrough the first switch.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments illustrated in the figures of the accompanying drawingsherein are by way of example and not by way of limitation. In thedrawings:

FIG. 1 is a circuit diagram of a conventional circuit having a trimmingcircuit;

FIG. 2A is a circuit diagram according one exemplary embodiment of thepresent invention;

FIG. 2B is a circuit diagram according one exemplary embodiment of thepresent invention;

FIG. 3 is I-V characteristics curves of a MOS transistor; and

FIG. 4 is a circuit diagram according to one exemplary embodiment of thepresent invention as implemented in an Ethernet system.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Terms such as “substantially,” “about,” “nearly,” “approximately” or thelike as used in referring to a relationship between two objects orvalues are intended to reflect not only an exact relationship but alsovariances in that relationship that may be due to various factors suchas common or accepted error tolerances, variations or the like. Itshould further be understood that although some values or otherrelationships may be expressed herein without a modifier, these valuesor other relationships may also be exact or may include a degree ofvariation due to various factors such as common or accepted errortolerances, risk tolerances, variations or the like.

FIG. 2A is a circuit diagram according to one exemplary embodiment ofthe present invention (“exemplary” as used herein referring to “servingas an example, instance or illustration”). Referring to FIG. 2A, acircuit 200 comprises an amplifier circuit 202 and a trimming circuit204. The amplifier circuit 202 includes an operational amplifier 206having a first (e.g., positive) input 208 for receiving input signals, asecond (e.g., negative) input 210 and an amplifier output 212. The inputsignals can be either voltage source or current source. In thisexemplary embodiment, the first input 208 is coupled to a voltage sourceV_(in). The output of the operational amplifier 206 is also in the formof a voltage V_(p). The trimming circuit 204 is coupled between theamplifier output 212 and a load impedance R_(load). The operationalamplifier 206 may be a non-inverting operational amplifier, an invertingoperational amplifier or other circuits comprising two inputs and oneoutput where one input is configured to receive the input signals andthe output is coupled back to one of the two inputs. In the exemplaryembodiment as shown in FIG. 2A, the operational amplifier 206 is anon-inverting operational amplifier where the input signal is a positivesignal received by the first input 208. In another exemplary embodimentwith reference to FIG. 2B, the operational amplifier 206 is an invertingoperational amplifier where the input signal is received by the secondinput 210. The amplifier output 212 is coupled to the negative inputwhich is the second input 210 as shown in FIG. 2A and FIG. 2B. Thedescription below describes the operation of circuit illustrated in FIG.2A. The circuit in FIG. 2B operates in a similar way in terms ofcorrecting the linearity of the output signals Vout.

As shown in FIG. 2A, the trimming circuit 204 includes a terminationresistor R_(S0) coupled in parallel with at least one trimming resistor,for example, a trimming resistor R_(S1). The termination resistor R_(S0)is coupled to a first switch S₀ in series. The trimming resistor R_(S1)is coupled to a second switch S₁ in series. More generally, if there area plurality of trimming resistors R_(S1), R_(S2) . . . R_(SN) coupled inparallel between the amplifier output 212 and the load impedanceR_(load), each trimming resistor may be coupled in series to arespective second switch. For example, trimming resistors R_(S1), R_(S2). . . R_(SN) may be coupled in series to respective second switches S₁,S₂ . . . S_(N).

The amplifier output 212 is connected back to the negative input 210through the first switch S₀ and a feedback circuit (not numbered). Asshown, the feedback circuit may be formed by a feedback resistor R_(f)and an input resistor R_(g). One end of the feedback resistor R_(f) iscoupled to a terminal of the first switch S₀ opposite the terminal ofthe first switch S₀ to which the amplifier output 212 is coupled. Theother end of the feedback resistor R_(f) is coupled to the negativeinput 210 of the amplifier 206. The negative input 210 is also coupledto virtual ground through the input resistor R_(g).

In an exemplary embodiment, the first switch S₀ may be the same type asthe second switches S₁, S₂ . . . S_(N). For example, both of the firstswitch S₀ and the second switch S₁ may be voltage-controlled switches.More specifically, the first switch S₀ and the second switch S₁ may beMOS transistors, which consist of three terminals, namely a source,drain and gate. In another exemplary embodiment, each of the firstswitch S₀ and the second switch S₁ may be a transmission gate comprisinga pair of complementary MOS transistors. Operation modes of MOStransistors depend on the terminal voltages V_(S), V_(D), and V_(G) atthe source, drain and gate, respectively. When gate-to-source voltageV_(GS) is greater than threshold voltage V_(T), a channel may be formedbetween the source and the drain. When the channel is pinched off nearthe drain, i.e., V_(GS)>(V_(GS)−V_(T)), the MOS transistor may becompletely or near-completely conducted, and the current I_(DS) flowingin the channel may be saturate or near saturate. The saturate currentI_(DS) may be calculated with the I-V characteristics formula:

${\frac{1}{2}\mu\;{C_{ox}( \frac{W}{L} )}( {V_{GS} - V_{T}} )^{2}},$where μ is the mobility, C_(ox) is the capacitance per unit area of thegate, and W and L are the width and length of the channel. The I-Vcharacteristics curves of a MOS transistor according to one exemplaryembodiment are shown in FIG. 3.

The I-V characteristics equation and FIG. 3 show that switches exhibit avariation in the nonlinearity resulting from device imperfections andvariances of the fabrication process. To cancel or reduce thenonlinearities of the switches which may cause mismatches in thecurrents, referring back to FIG. 2A, the sizes of the first switch S₀and the second switch S₁ may be pre-determined in accordance with thefollowing:

$\frac{W_{0}/L}{W_{1}/L} = {\frac{R_{S\; 1}}{R_{S\; 0}}.}$As the result, the on-resistance R_(on) _(—) _(S0) of the first switchS₀ and the on-resistance R_(on) _(—) _(S1) of the second switch S₁ maybe represented by:

$\frac{R_{{on\_ S}\; 1}}{R_{{on\_ S}\; 0}} = {\frac{R_{S\; 1}}{R_{S\; 0}}.}$If a variable c is defined as

${\frac{R_{{on\_ S}\; 1}}{R_{{on\_ S}\; 0}} = {\frac{R_{S\; 1}}{R_{S\; 0}} = c}},$the following relationships may be obtained: R_(on) _(—) _(S1)=c·R_(on)_(—) _(S0) and R_(S1)=c·R_(S0).

In addition to the foregoing, a voltage V_(A) at node A can becalculated as a function of the on-resistance of the first switch asfollows:

$\begin{matrix}{V_{A} = {{( \frac{R_{S\; 0}}{R_{{on\_ S}\; 0} + R_{S\; 0}} ) \cdot V_{p}} + {( \frac{R_{{on\_ S}\; 0}}{R_{{on\_ S}\; 0} + R_{S\; 0}} ) \cdot V_{outp}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$where V_(p) is the output voltage of the operational amplifier 206 andV_(outp) is the voltage at a node 222. In a similar manner, voltageV_(B) at node B can be calculated as follows:

$\begin{matrix}{V_{B} = {{( \frac{R_{S\; 1}}{R_{{on\_ S}\; 1} + R_{S\; 1}} ) \cdot V_{p}} + {( \frac{R_{{on\_ S}\; 1}}{R_{{on\_ S}\; 1} + R_{S\; 1}} ) \cdot V_{outp}}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$Then, by replacing R_(S1) with c·R_(S0) and R_(on) _(—) _(S1) withc·R_(on) _(—) _(S0) in equation (2), the following may be obtained:

$V_{B} = {{{( \frac{c \cdot R_{s\; 0}}{{c \cdot R_{{on}\; 0}} + {c \cdot R_{s\; 0}}} ) \cdot V_{p}} + {( \frac{c \cdot R_{{on}\; 0}}{{c \cdot R_{{on}\; 0}} + R_{S\; 0}} ) \cdot V_{outp}}} = {{{( \frac{R_{S\; 0}}{R_{{on}\; 0} + R_{S\; 0}} ) \cdot V_{p}} + {( \frac{R_{{on}\; 0}}{R_{{on}\; 0} + R_{S\; 0}} ) \cdot V_{outp}}} = V_{A}}}$

As derived above, in operation, voltage V_(A) at the node A may equal orapproximately equal voltage V_(B) at the node B. Because negativefeedback may improve the linearity of amplifiers, mismatch effects onthe trimming circuit 204 caused by the nonlinearity of the first switchS₀ may be reduced or eliminated by connecting the amplifier output 212back to the negative input 210 through the first switch S₀. As theresult, a current I_(A) flowing through the termination resistor R_(S0)may be approximately a linear current. V_(A) at the node A may betherefore viewed as a linear voltage. In addition, because V_(A) mayequal V_(B), the second switch S₁ may be considered as a part of thefeedback circuit, the effect on the trimming circuit 202 caused by thenonlinearity of the second switch S ₁ may be accordingly canceled. V_(B)at node B may also be viewed as a linear voltage, and a current I_(B)flowing through the trimming resistor R_(S1) may be a linear current.Thus, the total resistance of the trimming circuit 204 may equal orapproximately equal to R_(S0)//R_(S1) and may not be affected by thenonlinearities of the first switch S₀ and the second switch S₁. CurrentI_(LD) following through the load impedance R_(load) may beapproximately the sum of the current I_(A) and the current I_(B), whichmay be a linear current in the same manner as I_(A) and I_(B). Likewise,the voltage across the load impedance R_(load) may be a linear voltage.

FIG. 4 is a circuit diagram according to one exemplary embodiment of thepresent invention as implemented in an Ethernet system. Referring toFIG. 4, a circuit 400 comprises an amplifier circuit 402 including twooperational amplifiers 406A and 406B. The amplifier circuit 402 canprovide either voltage source or current source. In this exemplaryembodiment, the positive input of each of the operational amplifiers406A and 406B is coupled to a voltage source V_(in). Negative inputs ofthe two operational amplifiers 406A and 406B are connected via an inputresistor R_(g). The circuit 400 further comprises a first trimmingcircuit 404A coupled between the output of the operational amplifier406A and a first terminal (shown as “txp”) of a load impedance (notnumbered), and a second trimming circuit 404B coupled between the outputof the operational amplifier 406B and a second, different terminal(shown as “txn”) of the load impedance. In other words, the firsttrimming circuit 404A is in parallel with the second trimming circuit404B. Each of the trimming circuits 404A and 404B includes a terminationresistor R_(S0) coupled in parallel with at least one trimming resistor.In this exemplary embodiment, each trimming circuit includes twotrimming resistors, R_(S1) and R_(S2). Each termination resistor R_(S0)is coupled to a respective first switch S₀ (S_(0A), S_(0B)) in series,and each trimming resistor is coupled to a respective second switch inseries. For example, the trimming resistor R_(S1) is coupled to thesecond switch S₁ (S_(1A), S_(1B)) in series, and the trimming resistorR_(s2) is coupled to the second switch S₂ (S_(2A), S_(2B)) in series.The circuit 400 further comprises an impedance trimming controller 407to control ON or OFF of the second switches S₁ and S₂, which are coupledto the respective trimming resistors in series.

In operation, the first switches S₀ coupled in series to the terminationresistors R_(s0) may be kept in a conducting state. To reduce oreliminate the nonlinearities caused by the second switches S₁, forexample, the output of the operational amplifier 406A may be connectedback to its negative input through the first switch S_(0A). Depending onthe impedance required by the system, the impedance trimming controller407 may be configured to change the status of the second switches S₁ toimplement the impedance requirement. For example, if a 50Ω impedance isrequired by the system, the second switch S_(1A) of the trimming circuit404A may be turned on. As the result, the total resistance of thetrimming circuit 404A may be

$\frac{1}{{{1/300}\Omega} + {{1/60}\Omega}},$or 50Ω.

Exemplary embodiments of the present invention also include a method forreducing impedance mismatches in a communications system. In oneexemplary embodiment, the method may be implemented in an Ethernetsystem. The method may include providing an operational amplifier havinga positive input configured to receive input signals, and that also hasa negative input and an amplifier output. The method may further includeproviding a trimming circuit coupled to the amplifier output. Thetrimming circuit includes a termination resistor coupled in parallelwith at least one trimming resistor. The method may also provide a firstswitch coupled to the termination resistor in series and a second switchcoupled to the trimming resistor such that a ratio of the first switchresistance to the second switch resistance approximately equals a ratioof the termination resistor resistance to the trimming resistorresistance. The amplifier output may be connected back to the negativeinput through the first switch.

It will be appreciated by those skilled in the art that changes could bemade to the examples described above without departing from the broadinventive concept. It is understood, therefore, that this invention isnot limited to the particular examples disclosed, but it is intended tocover modifications within the spirit and scope of the present inventionas defined by the appended claims. Although specific terms are employedherein, they are used in a generic and descriptive sense only and notfor purposes of limitation.

1. A circuit comprising: an amplifier circuit including an operationalamplifier, the operational amplifier having a first input configured toreceive input signals, and the operational amplifier also having asecond input and an amplifier output, wherein one of the first input orthe second input is a negative input; and a trimming circuit coupled tothe amplifier output, the trimming circuit including a terminationresistor coupled in parallel with at least one trimming resistor,wherein the termination resistor is coupled to a first switch in series,and the at least one trimming resistor is coupled to a second switch inseries, wherein the amplifier output is connected back to the negativeinput through the first switch, wherein the first switch includes twoends, one end connected to the amplifier output, and another endconnected to a feedback circuit, and wherein the first switch is alwaysin a conducting state.
 2. The circuit of claim 1, wherein the firstswitch comprises at last one metal-oxide-semiconductor transistor. 3.The circuit of claim 1, wherein the first switch comprises at least onevoltage-controlled switch.
 4. The circuit of claim 1, wherein the firstswitch comprises a transmission gate.
 5. The circuit of claim 1, whereina ratio of a resistance of the first switch to a resistance of thesecond switch approximately equals a ratio of a resistance of thetermination resistor to a resistance of the trimming resistor.
 6. Thecircuit of claim 1, wherein a total resistance of the trimming circuitapproximately equals a parallel resistance of the termination resistorand the trimming resistor.
 7. The circuit of claim 1, wherein the firstswitch and the second switch comprises the same type of switch.
 8. Acircuit comprising: an amplifier circuit including an operationalamplifier, the operational amplifier having a first input configured toreceive input signals, and the operational amplifier also having asecond input and an amplifier output, wherein one of the first input orthe second input is a negative input; and a trimming circuit coupled tothe amplifier output, the trimming circuit including a terminationresistor coupled in parallel with at least one trimming resistor,wherein the termination resistor is coupled to a first switch in series,and the at least one trimming resistor is coupled to a second switch inseries, such that a ratio of a resistance of the first switch to aresistance of the second switch approximately equals a ratio of aresistance of the termination resistor to a resistance of the trimmingresistor, wherein the first switch includes two ends, one end connectedto the amplifier output, and another end connected to a feedbackcircuit, and wherein the first switch is always in a conducting state.9. The circuit of claim 8, wherein the first switch comprises at leastone metal-oxide-semiconductor transistor.
 10. The circuit of claim 8,wherein the first switch comprises at least one voltage-controlledswitch.
 11. The circuit of claim 8, wherein the first switch comprises atransmission gate.
 12. The circuit of claim 8, wherein the circuit isconfigured to keep the first switch in a conducting state.
 13. Thecircuit of claim 8, wherein a total resistance of the trimming circuitapproximately equals a parallel resistance of the termination resistorand the trimming resistor.
 14. The circuit of claim 8, wherein theamplifier output is connected back to the negative input through thefirst switch and a feedback circuit.
 15. The circuit of claim 8, whereinthe first switch and the second switch comprises the same type ofswitch.
 16. A method comprising: providing an operational amplifier, theoperational amplifier having a first input configured to receive inputsignals, and the operational amplifier also having a second input and anamplifier output, wherein one of the first input or the second input isa negative input; providing a trimming circuit coupled to the amplifieroutput, the trimming circuit including a termination resistor coupled inparallel with at least one trimming resistor; and providing a firstswitch coupled to the termination resistor in series, and a secondswitch coupled to the at least one trimming resistor in series, suchthat a ratio of a resistance of the first switch to a resistance of thesecond switch approximately equals a ratio of a resistance of thetermination resistor to a resistance of the trimming resistor, whereinthe amplifier output is connected back to the negative input through thefirst switch, wherein the first switch includes two ends, one endconnected to the amplifier output, and another end connected to afeedback circuit, and wherein the first switch is always in a conductingstate.
 17. The method of claim 16 further comprising keeping the firstswitch in a conducting state.
 18. The method of claim 16, wherein atotal resistance of the trimming circuit approximately equals a parallelresistance of the termination resistor and the trimming resistor.